Process for the extension of microbial life and color life of fresh meat products

ABSTRACT

The microbial life and color life of fresh meat products are extended by placing fresh meat in a tray, sealing the fresh meat in the tray in an atmosphere that excludes oxygen and contains carbon monoxide, and applying ionizing radiation to the tray containing the fresh meat. The color life of the fresh meat in the tray may be controlled (such as to be equal to or less than the microbial life) by employing a permeable element and an impermeable element to seal the fresh meat in the tray. The impermeable element is removed at a time after ionizing radiation has been applied, so that the atmosphere that excludes oxygen and contains carbon monoxide is exchanged for ambient atmosphere, resulting in predictable limiting of color life of the fresh meat.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 60/781,529 filed Mar. 10, 2006 for EXTENDED STORAGE LIFE FRESH MEAT by B. Dalziel, V. Kennedy and S. Koenck.

INCORPORATION BY REFERENCE

The aforementioned U.S. Provisional Application No. 60/781,529 is hereby incorporated by reference in its entirety.

BACKGROUND

The present invention relates to a method of preserving the freshness and color life of meat products, and also to a method of matching the color life to the microbial life of the meat products.

An increasingly popular method of providing fresh meat in the retail environment is case-ready packaging, whereby the meat product is processed and packaged in a retail package by a meat packer that may be quite distant from the retail facility. Case-ready packaging is especially appealing to large retail organizations that specialize in highly efficient logistics and supply chain management, because it enables them to provide an otherwise relatively perishable product in their stores with minimum investment in special infrastructure. The challenge for these organizations, even with their world-class distribution capability, is shelf life. The time between processing fresh meat until the product is sold to the consumer averages over 10 days, and may be as high as 21 days.

It is quite difficult to provide good quality and good looking fresh meat products to consumers with a shelf life as high as 21 days. The dual challenges for shelf life of this magnitude are microbial growth and product color. Microbial growth is related to the geometric multiplication of spoilage bacteria that causes fresh meat products to spoil. Product color is related to the shift in color spectrum of the myoglobin constituents in fresh meats.

Meat scientists have researched the effect of the oxidative states of myoglobin on the color of fresh meat. Fully fresh, recently processed meat is dominated by the presence of oxymyoglobin, which has oxygen attached to the myoglobin base molecule and is bright pink-red in color in the so-called “bloomed” state. If fresh meat is placed in an oxygen-depleted environment, the oxygen is lost and the myoglobin turns to the deoxymyoglobin state, which takes on a deep purple-brown color. This product may still be very fresh, but its color is not familiar, and consumers may be reluctant to purchase product with this appearance. As fresh meat ages, the myoglobin progressively turns to the metmyoglobin state that is characterized by the brown color familiarly associated with old and possibly spoiled meat.

In the ongoing efforts to provide increased shelf life fresh meats, it has been learned that various modified atmosphere case-ready packages may be employed to mitigate the previously mentioned effects of microbial growth and color loss. Microbial growth in fresh meat can be suppressed by the use of atmospheres that spoilage bacteria do not thrive in. For example, many spoilage bacteria are aerobic, so they don't multiply well in an oxygen-deprived atmosphere. Case-ready modified atmosphere packaging (MAP) with a low oxygen atmosphere can delay spoilage in fresh meats. Unfortunately, low oxygen packaging causes the color of the fresh product to be dominated by deoxymyoglobin, with its characteristic purple-brown color. Case-ready MAP products with a high oxygen atmosphere can delay spoilage due to a high concentration of carbon dioxide, and the oxygen concentration extends the oxymyoglobin state to maintain the preferred pink-red color state. High oxygen MAP is generally more successful for delivery of case-ready fresh meat in the retail distribution marketplace than low oxygen MAP, but even with its better characteristics, it is generally limited to less than 10 days of shelf life.

In the ongoing search for methods of extending the shelf life of fresh meats, scientists have discovered that carbon monoxide is capable of extending color life. Carbon monoxide reacts with myoglobin in a way very similar to the way it reacts with hemoglobin—the basic oxygen and carbon dioxide carrying constituent in blood. Carbon monoxide has high affinity for the oxygen-carrying site in both hemoglobin and myoglobin, and binds approximately 240 times more strongly than oxygen or carbon dioxide. In the case of hemoglobin, a concentration of carbon monoxide may lead to death because the body may be deprived of necessary oxygen. In the case of myoglobin, the presence of carbon monoxide forms a special state of myoglobin labeled carboxymyoglobin. Carboxymyoglobin has a color spectrum that is nearly identical to oxymyoglobin, and causes meat to look identical to fresh meat that is dominated by the presence of oxymyoglobin. Meat processors and retailers have discovered that carbon monoxide as a MAP additive, even in low concentrations, lessens the color loss in fresh meat for significantly extended periods of time. A popular MAP atmosphere called “low oxygen” or “low ox” includes 70% nitrogen, 29.6% carbon dioxide and 0.4% carbon monoxide, with very minute levels of oxygen (if any) being present.

Iradiation technology has been utilized for decades as a means for reduction of micro flora in fresh and frozen food products. Applications of irradiation have included reduction of harmful food borne pathogens such as E. coli 0157:H7, Salmonella, and Listeria, as well as common spoilage bacteria for extension of shelf life. Irradiation has more recently been employed in systems and processes that package the irradiated meat products with modified atmosphere packaging to extend shelf life for case-ready retail applications.

While the combination of these technologies has generally extended the shelf life of fresh meat products, the desired 21 days (or more) of shelf life has been elusive, primarily due to color life limitations. What is needed is a process for extension of both the shelf (microbial) life and color life of fresh meat products.

SUMMARY

The microbial life and color life of fresh meat products are extended by placing fresh meat in a tray, sealing the fresh meat in the tray in an atmosphere that excludes oxygen and contains carbon monoxide, and applying ionizing radiation to the tray containing the fresh meat. The color life of the fresh meat in the tray may be controlled (such as to be less than or equal to the microbial life) by employing a permeable element and an impermeable element to seal the fresh meat in the tray. The impermeable element is removed at a time after ionizing radiation has been applied, so that the atmosphere that excludes oxygen and contains carbon monoxide is exchanged for ambient atmosphere, resulting in predictable limiting of color life of the fresh meat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a process for extending both the shelf life and color life of fresh meat products according to an embodiment of the present invention.

FIG. 2 is a cross section view of a prior art case-ready modified atmosphere package for meat.

FIG. 3 is a cross section view of a case-ready modified atmosphere package for meat according to one embodiment of the present invention.

FIG. 4 is a cross section view of a case-ready modified atmosphere package for meat according to another embodiment of the present invention

DETAILED DESCRIPTION

In order to extend both the shelf life and color life of fresh meat products, a process according to the present invention is employed. This process is illustrated in FIG. 1. Initially, meat is placed in a tray or another type of container. The meat-containing tray is then sealed with a modified atmosphere gas. For example, the modified atmosphere gas may be a low oxygen blend comprising 70% nitrogen, 29.6% carbon dioxide and 0.4% carbon monoxide, which is a gas blend that is generally recognized as safe (GRAS) by the Federal Food and Drug Administration. The sealed meat-containing, modified atmosphere tray is then irradiated with ionizing radiation. The benefit of applying ionizing radiation to fresh meat products in the absence of oxygen is explained in U.S. Pat. No. 6,885,011 (assigned to Mitec Incorporated of Cedar Rapids, Iowa). In the process of the present invention, the fresh meat product is maintained in an oxygen-free environment by virtue of the modified atmosphere present in the sealed tray that holds the fresh meat product. The chamber in which ionizing radiation is applied may also be kept substantially free of oxygen in some embodiments, as described in the aforementioned U.S. Pat. No. 6,885,011. After irradiating the fresh meat product in the oxygen-excluding tray, the product will experience extended microbial shelf life due to the effect of the ionizing radiation on food borne pathogens and spoilage bacteria in the fresh meat product. In addition, the color life of the fresh meat product will also be extended by the low oxygen, carbon monoxide-containing modified atmosphere that is provided in the sealed tray. In some scenarios, the microbial life and color life of the fresh meat product may extend as long as 90 days.

In one particular embodiment, the low oxygen, carbon monoxide-containing modified atmosphere is introduced into the tray containing the fresh meat product and is allowed to dwell there for an extended period of time before the fresh meat product is irradiated. Allowing the carbon monoxide of the modified atmosphere gas to dwell sets the color of the fresh meat product to a greater extent than would otherwise occur, so that a higher dose of irradiation can be applied to the fresh meat product without causing discoloration.

In some embodiments, it is desirable to control the length of the color life of fresh meat products to ensure that the color life fails before the microbial life, so that consumers have a reliable indication that the product may be spoiled and should not be consumed. Experience has shown that carbon monoxide MAP packaging may be so successful at preserving the color life of packaged fresh meat products such as ground beef, that in some cases these products may have a very wholesome looking fresh appearance, but may in fact be quite spoiled. This situation is addressed as discussed below with respect to FIGS. 2-4.

FIG. 2 is a cross section view of a prior art case-ready modified atmosphere package for meat. The tray 1 is typically formed of thermoplastic material such as styrofoam into a shape with a cavity, sidewalls and a flat upper lip surface 5 that serves as a sealing surface for a lid film 2. Lid film 2 is typically a clear plastic sheet material that allows a consumer to view the meat packaged inside the tray 1. Meat 3 may be ground meat of either loaf or patty form, a whole muscle cut of meat such as a steak, roast or filet, or the like. A headspace 4 above and around the meat provides a volume that may be filled with a gas combination different from ambient air to advantageously affect the storage characteristics of the meat packaged and sealed inside the tray.

The tray of FIG. 2 may be processed in a number of different ways. A typical process is to simply place a meat product in the tray and seal the tray with a modified atmosphere package (MAP) tray sealer (not shown). MAP tray sealers are well known in the meat packaging industry, and operate by removing the ambient atmosphere around the meat product in the tray, replacing the atmosphere around the meat product in the tray in the headspace 4 with a modified atmosphere, and applying and sealing the lid film 2 to the tray lip 5 to form a gas tight seal that persistently contains the modified atmosphere in the interior of the tray.

An alternate process for the tray of FIG. 2 is to place a meat product in the tray, expose the meat product to ionizing radiation in an open tray before affixing the lid film to the tray, preferably in an atmosphere that excludes oxygen as disclosed in U.S. Pat. No. 6,885,011 (assigned to Mitec Incorporated of Cedar Rapids, Iowa), and seal the tray with a modified atmosphere package (MAP) tray sealer (not shown). This alternate process has the benefit of reducing the population of micro flora including both food borne pathogens and spoilage bacteria, and will extend the storage life of meat products by delaying spoilage. Irradiation does not extend the color life of fresh meat products, however. The use of either high oxygen MAP or low oxygen with low concentration carbon monoxide MAP may be used to extend the color life of fresh meat products as described earlier. Unfortunately, there is no correlation or coordination between the spoilage life and the color life of the meat with this process, so it is possible that the color life might be longer than the spoilage life, which could lead to the undesirable situation where a consumer does not have a reliable indication that the product may be spoiled and should not be consumed.

Referring now to FIG. 3, a cross section view of a case-ready modified atmosphere package for meat according to the present invention is shown. The tray and meat product is identical to the prior art tray of FIG. 2, however, the lid film 12 is of a type that is gas permeable and allows the modified atmosphere gas constituents to slowly diffuse across the lid film barrier 12. A completely enclosing gas impermeable bag 10 is filled with an equivalent or complementary modified atmosphere 1, so there is an equilibrium state of gas transfer across the gas permeable film 12 as long as the enclosing impermeable bag 10 is in place. In an exemplary embodiment, the modified atmosphere gas placed inside the tray headspace 4 may be a low oxygen blend comprised of 70% nitrogen, 29.6% carbon dioxide and 0.4% carbon monoxide. In an exemplary embodiment, the gas blend placed inside the enclosing impermeable bag may be an identical low oxygen blend, so there will be no gas exchange across the impermeable lid film 12 as long as the enclosing impermeable bag is in place.

The tray of FIG. 3 may be processed in the same ways as tray of FIG. 2, including either ordinary MAP processing or processing with irradiation and MAP. If the modified atmosphere includes low concentrations of carbon monoxide, the color of the fresh meat product will be maintained for a longer period of time in the bright pink-red color preferred by consumers as long as the enclosing impermeable bag is in place.

In addition, the tray of FIG. 3 allows further processing alternatives to employed. In one embodiment, the meat product may be placed in tray 1 and permeable lid film 12 may be applied to tray 1 before ionizing radiation is applied (in an atmosphere that excludes oxygen, the benefits of which are disclosed in U.S. Pat. No. 6,885,011 referenced above). In this embodiment, no oxygen is present in the irradiation environment because of the fact that the atmosphere in tray 1 (sealed by permeable lid film 12) is a low oxygen blend that substantially excludes oxygen, and in some embodiments, the irradiation environment around the product is also controlled to exclude oxygen.

In another embodiment, the meat product may be placed in tray 1, permeable lid film 12 may be applied to tray 1, and gas impermeable bag 10 may enclose tray 1 and permeable lid film 12 before ionizing radiation is applied (in an atmosphere that excludes oxygen, as mentioned above). Again, in this embodiment, no oxygen is present in the irradiation environment because of the fact that the atmosphere in tray 1 (sealed by permeable lid film 12) is a low oxygen blend that substantially excludes oxygen, the atmosphere within gas impermeable bag 10 is filled with a low oxygen blend that substantially excludes oxygen, and the irradiation environment around the product is also controlled to exclude oxygen in some embodiments.

This configuration is ideal for extended storage life of fresh meat products. When it is time for the fresh meat product to be placed on the retail shelf, the enclosing impermeable bag is removed. At this point, the modified atmosphere begins to diffuse across the permeable lid film and the concentrations of MAP gas are exchanged with ambient air until they reach ambient air concentrations. Of particular interest is the concentration of carbon monoxide, as well as the characteristics of the fresh meat that has been exposed to the carbon monoxide. Research has shown that the carbon monoxide binds to myoglobin in a manner very similar to the way it binds to hemoglobin in blood. While the affinity of carbon monoxide to hemoglobin is approximately 240 times greater than oxygen, this binding is reversible with a half-life of approximately 4.5 hours.

In retail applications, it is desirable for the rate of release of carbon monoxide to be controlled to match the required display shelf life expected while being coordinated with the spoilage life of the fresh meat product. For example, in relatively high volume retail environments such as super stores, it may be desirable to have a week of display shelf life. To achieve one week of display shelf life, the gas permeable lid film should be selected so that enough carbon monoxide is exchanged with the ambient atmosphere that the carbon monoxide level is below the color maintenance level 7 days after the enclosing impermeable bag is removed. Other retail applications such as convenience stores may require longer display shelf life, such as two weeks. In such applications, the gas permeable lid film should be selected with a lower permeability to cause a slower exchange of carbon monoxide with the ambient atmosphere.

FIG. 4 is a cross section view of an alternate version of the extended storage life fresh meat tray according to the present invention. Rather than placing the entire tray 1 inside an enclosing gas impermeable bag 10 of FIG. 3, a peelable gas impermeable film 30 is placed immediately over a gas permeable lid film 22 that is sealed to the tray lip 5. In some embodiments, gas permeable lid film 22 and peelable gas impermeable film 30 may be dual layers of a single film. This package has the benefit of a standard tray configuration, which is convenient for processing, shipping and handling. The film peeling step involves some additional labor, but this labor is generally similar to removing the gas impermeable bags 10 of FIG. 3. The primary disadvantage of the possibility that store personnel might forget to remove the peelable gas impermeable film may be mitigated by placing a label on the exterior of the impermeable film that is visible to store personnel as an indication that the film has not been properly removed.

As in the tray configuration of FIG. 3, the tray of FIG. 4 may be processed with or without irradiation. All of the processing options discussed above with respect to the tray configuration of FIG. 3 are also available for the tray configuration of FIG. 4. The tray of FIG. 4 may also include any variety of MAP gas. An exemplary MAP gas is a blend of 70% nitrogen, 29.6% carbon dioxide and 0.4% carbon monoxide. This MAP gas blend can maintain the color of fresh meat products for up to 40 days or more. Irradiation processing can provide microbial life up to 50 days or more.

Management of shelf life expiration of fresh meat products processed and distributed according to the present invention is essentially the same as any other perishable product with a posted expiration date. The expiration date affixed or printed on the trays is determined by the microbial life of the product. The color life of fresh meat in storage according to the present invention is generally longer than the microbial storage life of the product until the gas impermeable bag or film is removed. Once this step is completed, the color life of the product is very predictably limited so that the color of the product becomes a meaningful indication of the state of freshness of the product. Specifically, in many embodiments it is desirable for the permeability of the permeable film to be selected in a way that provides a color life of a particular length of time, and for the dose of ionizing radiation to be selected in a way that provides a microbial life of a particular length of time that has a known relationship to the color life. For example, in some embodiments, it is desirable for the color life to match the microbial life. In other embodiments, it is desirable for the color life to be slightly less than the microbial life. The permeability of the permeable film over the tray and the dose of ionizing radiation are controllable variables that allow this correspondence between microbial life and color life to be achieved.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A method of providing fresh meat products, comprising: placing fresh meat in a tray and sealing the fresh meat in the tray in an atmosphere that excludes oxygen and includes carbon monoxide; and applying ionizing radiation to the tray containing the fresh meat.
 2. The method of claim 1, wherein the atmosphere that excludes oxygen further includes nitrogen and carbon dioxide.
 3. The method of claim 2, wherein the atmosphere that excludes oxygen consists of 70% nitrogen, 29.6% carbon dioxide and 0.4% carbon monoxide.
 4. The method of claim 1, wherein the step of applying ionizing radiation to the tray containing the fresh meat is performed in an environment that is controlled to exclude oxygen.
 5. The method of claim 1, further comprising: unsealing the fresh meat in the tray in a retail environment after ionizing radiation has been applied.
 6. The method of claim 5, wherein the step of sealing the fresh meat in the tray comprises: applying a permeable lid film to the tray to secure the fresh meat in the tray; and enclosing the tray and permeable lid film in an impermeable bag.
 7. The method of claim 6, wherein the step of unsealing the fresh meat in the tray comprises: removing the tray containing the fresh meat secured by the permeable lid film from the impermeable bag.
 8. The method of claim 7, wherein the permeable lid film has a permeability that provides a color life to the fresh meat in the tray that has a known correspondence to a microbial life of the fresh meat in the tray that is based on a dose of ionizing radiation applied to the fresh meat.
 9. The method of claim 5, wherein the step of sealing the fresh meat in the tray comprises: applying a permeable lid film to the tray to secure the fresh meat in the tray; and applying a peelable impermeable film over the permeable lid film.
 10. The method of claim 9, wherein the step of unsealing the fresh meat in the tray comprises: peeling the impermeable film off of the permeable lid film.
 11. The method of claim 10, wherein the permeable lid film and the peelable impermeable film are multiple layers of a single film.
 12. The method of claim 10, further comprising: applying a notification label on the peelable impermeable film to provide notification that the peelable impermeable film has not yet been removed.
 13. The method of claim 10, wherein the permeable lid film has a permeability that provides a color life to the fresh meat in the tray that has a known correspondence to a microbial life of the fresh meat in the tray that is based on a dose of ionizing radiation applied to the fresh meat.
 14. A method of providing fresh meat products, comprising placing fresh meat in a tray and sealing the fresh meat in the tray in an atmosphere that excludes oxygen and includes carbon monoxide; applying ionizing radiation to the tray containing the fresh meat; and distributing the fresh meat in the tray for sale to consumers in a retail environment.
 15. The method of claim 14, further comprising: unsealing the fresh meat in the tray in the retail environment after ionizing radiation has been applied.
 16. The method of claim 15, wherein the step of sealing the fresh meat in the tray comprises: applying a permeable lid film to the tray to secure the fresh meat in the tray; and providing an impermeable seal over the permeable lid film.
 17. The method of claim 16, wherein the step of unsealing the fresh meat in the tray comprises: removing the impermeable seal over the permeable lid film.
 18. The method of claim 17, wherein the permeable lid film has a permeability that provides a color life to the fresh meat in the tray that has a known correspondence to a microbial life of the fresh meat in the tray that is based on a dose of ionizing radiation applied to the fresh meat.
 19. The method of claim 16, wherein the impermeable seal over the permeable lid film comprises an impermeable bag enclosing the tray and the permeable lid film.
 20. The method of claim 16, wherein the impermeable seal over the permeable lid film comprises a peelable impermeable film over the permeable lid film. 